Mass

Size

Discovery Methods

NGC 4253

Like poorly cooked oatmeal, the accretion disks that encircle many supermassive black holes are lumpy.

An accretion disk contains gas that is spiraling toward the black hole at a large fraction of the speed of light. The gas can come from passing stars or gas clouds, or can be the remains of pulverized asteroids or other bodies. The flow into the disk isn't always even, so the disk is sometimes lumpy.

And sometimes the disk can create its own lumps. That was the case with the black hole at the center of the small spiral galaxy NGC 4253. In the early 2000s, observations by Europe's XMM-Newton X-ray space telescope revealed three clumps of gas in the black hole's accretion disk. Each consisted of iron that had been heated to millions of degrees.

The clumps were about as far from the black hole as the planet Jupiter is from the Sun, roughly half-a-billion miles (800 million km). Yet while Jupiter takes 12 years to orbit the Sun, the clumps of matter in the accretion disk, accelerated by the black hole's overwhelming gravity, completed a circuit every 27 hours, giving them an orbital speed of 20,000 miles (30,000 km) per second -- roughly 10 percent of the speed of light.

Using the orbital speed of these clumps, along with other measurements, allowed the astronomers who studied XMM-Newton's observations to calculate the black hole's mass at roughly 20 million times the mass of the Sun, making it about as big as Mercury's orbit around the Sun.

The astronomers suggested that the clumps were created by magnetic fields produced within the accretion disk itself.

The black hole also has been detected through water masers -- intense beams of microwaves from pockets of water molecules outside the accretion disk. Energy from the disk "excites" the molecules, causing them to produce energy that can be detected by ground-based radio telescopes. The motion of the water masers reveals their orbital speed, which astronomers can use to calculate the mass of the object the masers are orbiting.